Sometimes, the factions build secret bases in secluded areas of the planet, where they can perform any activity that's best left unknown to the population at large (and their competition). However, this secluded location gives us an opportunity to strike, loot the place and disappear into the sunset before massive reinforcements can arrive. On the other hand, each Secret Base that exists steadily undermines our position. A Secret Base is bound to be filled with loot, information (which we can use to blackmail people and otherwise increase our global standing), and is a prime place to look for the highest-ranking members of a faction. The final stages of building a base are a special event - they're signified with massed Supply Ship, Frigate and Cruiser activity. The window of opportunity to strike is short, though, no more than a day.